Anti brake-lock or vehicle anti-skid systems utilize sensors coupled to the wheels. Such sensors are subject to mechanical disturbances, particularly vibrations, and may also respond by spurious signals to uncontrollable conditions in the transducer elements, such as out-of-round conditions of transducer components, and the like. Consequently, malfunction or interference with proper, controlled operation of the anti-skid system may occur. Vibrations and out-of-round conditions, particularly, may cause interference or disturbance signals which are within the frequency and/or amplitude range of the actual signal which is intended to be derived. If such disturbance signals are, erroneously, evaluated as actual wheel signals, malfunction of the anti-skid system may occur.
It has previously been proposed--see German Patent Disclosure Document DE-OS 17 80 062--to utilize filter circuits coupled to a speed transducer, in which the time constants of the filters are suitably selected to exclude, as far as possible, disturbance signals. Later, it was proposed--see German Patent Disclosure Document DE-OS 19 16 518 (based on U.S. application Ser. No. 716,709, filed Sept. 23, 1968, ATKINS, assigned Kelsey-Hayes Co.)--to construct filters which have low-pass characteristics utilizing an R/C series circuit arrangement.
It has been found that filters may, in connection with the remaining circuits in which they are used, form oscillatory systems if output signals from wheel rotation transducers are subjected to vibration or other recurring disturbances. If the circuit becomes oscillatory, disturbance and noise signals will be enhanced, which, in spite of other precautions taken to exclude disturbance signals, may lead to erroneous response, and hence malfunction of the anti-skid system.